Heretofore, heat-sensitive recording materials have been well known, in which colorless or pale-colored basic dyes and organic or inorganic color developing materials are brought into contact with each other by the application of heat to produce recorded images by utilizing the color reaction therebetween as disclosed in, for example, U.S. Pat. No. 3,539,375.
Recent remarkable advances in heat-sensitive recording systems have permitted high-speed operation of various kinds of apparatus utilizing a thermal head, such as heat-sensitive facsimiles and heat-sensitive printers. For example, modern heat-sensitive facsimiles can transmit a printed page of A4 size paper (210.times.297 mm) in 20 seconds, and modern heat-sensitive printers can print 120 or more letters per second. With the development of such high-speed facsimiles and printers, it is now required for heat-sensitive recording materials which are used in the high-speed facsimiles and printers to have a high recording sensitivity (dynamic recording characteristics). Furthermore, the materials must cause neither static recording within a low temperature range (60.degree. C. to 70.degree. C.) nor piling due to the attachment of tailings.
As the fields in which heat-sensitive recording materials are used expand, they are exposed to a greater chance of direct contact with fingers or contact with plastic films. This results in the recorded image undesirably losing or fading color by sebum of human being or reaction with the plasticizer in the plastics. To avoid such problem, the heat-sensitive recording material must have resistance to fingerprint and plasticizer.
Some conventional high sensitivity heat-sensitive recording materials are prepared using combinations of dyes, phenol compounds such as 4,4'-isopropylidenediphenol, and sensitizers such as stearic acid amide. In a recording layer of such high sensitivity heat-sensitive recording materials, the sensitizer and phenol compound are compounded in a proportion ranging between 2 and 5 per part by weight of the dye. These heat-sensitive recording materials, however, have disadvantages in that the so-called static recording characteristics within a low temperature range (60.degree. C. to 70.degree. C.) occurs as the recording sensitivity is increased, and the resistance to fingerprint and plasticizer is poor.
It has therefore been desired to develop heat-sensitive recording materials which are well-balanced in the quality thereof, i.e., resistance to fingerprint and plasticizer, and are suitable for use in high-speed recording.